The double-edged sword of potassium and sodium fertilization in xylem embolism resistance of two Eucalyptus species under drought stress

Sodium (Na+) is a beneficial element for most plants and may replace potassium (K+) in osmoregulatory process to a certain extent, increasing plant water use efficiency. Thus, understanding coordinated mechanisms underlying the combined use of K+ and Na+ in tree drought tolerance is a key challenge for forestry in dealing with productivity and water limitations. A pot experiment with three ratios of K/Na (K-supplied, partial K replacement by Na, and K-deficient plants) and two water regimes, well-watered (W+) and water-stressed (W−), was conducted on saplings of two Eucalyptus species with contrasting drought sensitivities. We evaluated the point of stomatal closure (Pgs90), xylem water potential at 12, 50, and 88% embolized xylem area (P12, P50, P88), hydraulic safety margin, leaf gas exchange (A, E, gs, and dark respiration), pre-dawn and midday leaf water potential (ΨPD and ΨMD), long-term water use efficiency (WUEL) and total dry mass. Partial K replacement by Na increased leaf gas exchange, WUEL, and total dry mass, while Pgs90, P12, P50, P88, and ΨMD decreased (were more negative), compared with plants exclusively supplied with K and K-deficient plants of both species. Fertilized plants had narrower hydraulic safety margins than K-deficient plants, indicating that these Eucalyptus species adopt the functional adaptive strategy of operating close to their hydraulic limits to maximize carbon uptake while increasing the risk of hydraulic failure under drought stress.